A conventional amplifier circuit having a switch has the structure shown in FIG. 1. QA0 and QB0 are mute-mode NPN transistors which have their emitters connected together, thus forming a mute-mode differential pair. QA1 and QB1, QA2 and QB2, . . . QAn and QBn are input NPN transistors, every two of which have their emitters coupled together, thus forming an input differential pair. The input transistors QA1, QA2, . . . QAn have their collectors connected together, forming a node (the node being denoted by A). The input transistors QB1, QB2, . . . QBn have their collectors connected together, forming a node (the node being denoted by B).
The bases of the input transistors QA to QAn are connected to a reference voltage terminal V.sub.REF by input-biasing resistors R1 to Rn, respectively, and are also connected to input terminals IN1 to INn, respectively. The bases of the input transistors QB1 to QBn are connected to a negative feedback terminal NF.
The base of the mute-mode transistor QA0 is connected to the reference voltage terminal V.sub.REF. The base of the transistor QB0 is coupled to the negative feedback terminal NF.
The circuit has a switch SW which can be connected to, and thus selects, the emitter node of the mute-mode transistors QA0 and QB0, that of the input transistors QA1 and QB1, that of the input transistors QA2 and QB2, that of the input transistors QAn and QBn, or that of the two input transistors forming any other intervening pair. A constant-current source I1 is connected between the selected emitter node and the ground potential terminal GND.
The collectors of NPN transistors Q1 and Q2, which form a current mirror, are connected to the nodes A and B, respectively. A resistor RE1 is connected between the emitter of the transistor Q1 and the a Vcc power-supply terminal. A resistor RE2 is connected between the emitter of the transistor Q2 and the Vcc power-supply terminal. The node B is connected to an output terminal OUT by an emitter follower circuit composed of an NPN transistor Q3 and a constant-current source I2 connected to the emitter of the transistor Q3. A feedback circuit comprised of resistors RNF1 and RNF2 and a capacitor CNF is connected between the output terminal OUT and the negative feedback terminal NF.
Input terminals IN1 to INn are coupled to input-signal sources S1 to Sn by coupling capacitors C1 to Cn, respectively.
In the amplifier circuit described above, which has the switch SW, the input-transistors pairs (QA1, QB1), (QA2, QB2), . . . (QAn, QBn) amplify the input signals input from the input-signal sources S1 to Sn to the input terminals IN1 to INn. The output of the input-transistor pair, which has been selected by the switch SW, is supplied to the output terminal OUT via the emitter follower circuit. The gain GV of the amplifier circuit having the switch SW is determined by the constant of the feedback circuit, as is represented by the following equation: EQU GV=(RNF1+RNF2)/RNF1 (1)
When the switch SW selects the mute-mode transistor pair (QA0, QB0), the gain GV is "0," and no outputs will be supplied from the amplifier circuit.
As has been described, the amplifier circuit has coupling capacitors C1 to Cn and an input-biasing resistors R1 to Rn. These elements are used for the purpose of preventing the DC potential of the output signal from changing when the switch SW is changed over. Were the input signals input directly to the bases of the input transistors QA1 to QAn, the DC potential of the output signal would vary when the switch SW is changed over if the DC potentials V1 to Vn of the input signals are different. The use of the coupling capacitors C1 to Cn and the input-biasing resistors R1 to Rn, however, means an increase in the number of elements forming the amplifier circuit.
Further, to form a mute-mode pair, the mute-mode transistors QA0 and QB0 are indispensable.
The gain GV of the amplifier circuit having the switch SW is set at a fixed value. In other words, it cannot be set at any value suitable for each input signal.
The conventional amplifier circuit having a switch has three problems. First, it comprises many elements. Second, it needs two mute-mode transistors for constituting a mute-mode pair. Third, its gain cannot be individually set for each input signal.
The present invention has been made in order to solve the problems described above. Its object is to provide an amplifier circuit having a switch, which comprises a relatively small number of elements, can have a gain individually set for each input signal, and can operate in mute mode without requiring mute-mode transistors.